Lithographic exposure apparatus (or exposure tools, for short) are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus, used for transfer of a pattern from a reticle onto a substrate of interest (interchangeably—a target substrate or a wafer, such as a semiconductor wafer during the semiconductor processing) includes an illumination source, a reticle stage assembly (that positions a reticle within the apparatus), an optical assembly containing the so-called projection optics, and a wafer stage assembly (that positions the target substrate or a wafer). The exposure apparatus also includes a measurement system (that monitors positions of the reticle and the target substrate) that employs an encoder head, and a control system that governs operations of various assemblies to adjust, when required, mutual positioning of the reticle and the target substrate. The geometrical features of patterns transferred from the reticle onto the target substrate are extremely small, which imposes extremely tight requirements on precise positioning of the target substrate and the reticle to manufacture high quality patterned semiconductor wafers.
Accuracy of the measurement system employed by the exposure apparatus constantly requires improvement (which is partly driven by advances in design of an exposure tool), while relatively small size, simplicity of construction, a need for reduced number of moving parts and high sensitivity remain as practical limitations.